Magnetoresistive random access memories (MRAMs) combine magnetic components to achieve non-volatility, high-speed operation, and excellent read/write endurance. In a standard MRAM device, such as that illustrated in FIG. 1 for a single bit, information is stored in the magnetization directions (illustrated by arrows) of individual magnetic tunnel junctions (MTJs) 102. MTJ 102 generally includes an insulating tunnel barrier 106 between two ferromagnetic layers: free ferromagnetic layer (or simply “free magnet”) 104, and fixed ferromagnetic layer (or “fixed magnet”) 108. In a standard MRAM, the bit state is programmed to a “1” or “0” using applied magnetic fields 114 and 116 generated by currents flowing along adjacent conductors—e.g., orthogonally-situated digit lines 118 and bit lines 110. The applied magnetic fields 114 and 116 selectively switch the magnetic moment direction of free magnet 104 as needed to program the bit state. When layers 104 and 108 are aligned in the same direction, and a voltage is applied across MTJ 102 (e.g., via isolation transistor 120 having a suitably controlled gate 121), a lower resistance is measured than when layers 104 and 108 are set in opposite directions.
In spin-transfer MRAM devices, such as that shown in FIG. 2, the bits are written by forcing a current directly through the stack of materials that make up the MTJ 102 (e.g., via current 202 controlled via isolation transistor 120). Generally speaking, the write current IDC, which is spin polarized by passing through one ferromagnetic layer (104 or 108), exerts a spin torque on the subsequent layer. This torque can be used to switch the magnetization of free magnet 104 between two stable states by changing the write current polarity. Spin-transfer MRAMs are advantageous in that they provide greater density with lower power consumption.
The relatively high current density traveling through the bit means that the resistance-area product (RA-product) of the tunnel barrier layer, which dominates the device-specific resistance, should be low—i.e., less than about 30Ωμm2. The voltage required to drive a high enough current density through the tunnel barrier to cause a magnetization reversal in a bit which is stable against thermally-induced magnetization reversal is very close to the tunnel barrier breakdown voltage. While various known tunnel barrier structures provide suitable magnetoresistance and spin torque effects, the required switching voltage of such tunnel barriers remains very close to the tunnel barrier breakdown voltage, resulting in a high percentage of devices that breakdown before switching.
It is therefore desirable to provide improved tunnel barrier structures for sensors and spin-transfer MRAM devices, particularly low resistance-area product MTJs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.